The present invention relates to a method for measuring the amount of gas that leaks through sealed packages. More specifically, the invention relates to gas leakage through packages that have been sealed by a cover of porous material fabricated as a mat of polyethelene fibers. This material acts as a permeable membrane to gases, but an impermeable membrane to bacteria. The membrane comprises a layer having pores which provide a tortuous path to the passage of bacteria; the material is commonly sold under the trademark designation xe2x80x9cTYVEC.xe2x80x9d The packages which use this material are typically semi-rigid plastic cases which protect medical devices and appliances after manufacture and before actual use.
The invention relates to U.S. Pat. No. 5,939,619, issued Aug. 17, 1999, entitled xe2x80x9cMethod and Apparatus for Detecting Leaks in Packaging,xe2x80x9d and U.S. Pat. No. 6,050,133, issued Apr. 18, 2000, entitled xe2x80x9cMethod and Apparatus for Detecting Leaks in Packaging.xe2x80x9d Both of these patents are owned by the assignee of the present invention. The present invention permits a much faster measurement of leakage than either of the foregoing prior art patents, with high sensitivity and less possibility of destroying the test package.
Gas sterilization is widely used for medical devices that must be sterile at the time of use, but cannot be subjected to sterilization by the application of high temperatures. Examples of such medical devices include cardiac pacemakers and catheter-based monitoring devices such as blood pressure probes. Typically, the medical device is sealed within a package that is permeable to gases but impermeable to bacteria. The package is then placed in a gas sterilization chamber, and a sterilizing gas such as ethylene oxide is introduced into the gas-permeable package to achieve sterilization. The sterilizing gas is then removed from the package, leaving the interior of the package sterile and non-toxic.
In a typical design, the medical device is placed within a thermoformed rigid plastic tray equipped with a flat sealing flange. A sheet of gas-permeable membrane, such as DuPont TYVEK(copyright) 1073-B (medical grade) brand membrane, which is available from E.I. duPont de Nemours and Co., is then sealed to the sealing flange, typically by using an adhesive. The integrity of the seal is critically important to maintaining sterility. Leaks can result from incorrect setting of parameters in the automated sealing process, or from physical defects such as burrs on the face of the sealing equipment.
According to the known practice described in the prior art patents listed herein, a temporary barrier is formed over the gas-permeable layer, wherein the temporary barrier has an aperture with the gas-permeable layer to temporarily seal the gas-permeable layer except where the aperture is located. A tracer gas is applied under low pressure through the aperture so that it can enter into the interior chamber of the package. The entire package is placed into a larger sealed second chamber, and the concentration of tracer gas in the second chamber is measured, outside the package, to thereby measure the amount of tracer gas which has leaked through the package, presumably via leaks in the sealing flange, although leakage can also occur through pinhole defects in the plastic tray itself.
One problem which exists with the methods described in the foregoing patents is that necessarily only very small concentrations of tracer gas can be introduced into the package, and therefore only very, very small concentrations of tracer gas must be measured to determine whether there is a leakage condition. The typical package used for containing medical devices is formed from rigid plastic material, and only about 3 to 5 psig pressure can be applied before the pressure will burst the seals of the package. This means that only about 2 cc of tracer gas can be delivered into a package having a volume of about 200cc, which provides only a low concentration of tracer gas into the package, and a much lower concentration of leakage tracer gas for measurement outside the package. This increases the sensitivity requirements of the measuring instruments and increases the time required for a successful measurement to be made; both are factors which increase the cost of testing.
The present invention comprises a method for testing sealed packages of the type described herein for leakage, according to a more efficient and faster process than is known in the prior art. The method requires a barrier layer to be overlaid on the gas-permeable membrane of the package, the barrier layer having two apertures to the gas-permeable membrane. The package is placed into a test chamber with the two apertures protruding outside the chamber, the chamber also having a carrier gas inlet and a carrier gas outlet. The carrier gas inlet is initially blocked and a slight vacuum is applied to the carrier gas outlet. After a few seconds, the tracer gas flow is started through the package apertures, passing through the permeable membrane, to thereby provide a continuous flow of tracer gas through the package undergoing test. The carrier gas outlet is then connected to a detector and the carrier gas inlet is opened to carrier gas at ambient pressure, resulting in a surge of carrier gas through the test chamber, which picks up trace amounts of the tracer gas, if any tracer gas leaks from the package, and the carrier gas, with its contained tracer gas, is conveyed to a suitable detector, capable of providing a measurement signal corresponding to the amount of tracer gas which is detected.
It is a principal object and advantage of the present invention to provide a method for measuring leakage through packages under faster test conditions than have heretofore been known.
It is another object and advantage of the present invention to provide a leakage test method which does not destroy the package being tested.
Other and further objects and advantages of the invention will become apparent from the following specification and claims and with reference to the appended drawings.